The present invention relates generally to the field of optical communications, and more specifically to the field of lasers.
The frequency stabilization of laser devices is highly valued especially in optical communication systems using dense wavelength division multiplexing (DWDM). Such systems presently envision using eighty or more wavelengths with approximately 50 GHz channel spacings which require laser frequency stabilization within, at the most, a few gigahertz in order to avoid, for example, channel interference. Frequency stabilization has typically been achieved using a variety of different techniques.
One conventional technique uses a laser, a piezoelectric device, a modulator, a Fabry-Perot etalon (FP), a photodiode and a balanced mixer. The output of the laser is fed into the FP. The modulator modulates the piezoelectric device which changes the effective cavity length of the FP. In the art, this is referred to as dithering the laser frequency. The output of the FP is received by the photodiode which creates an electrical signal. The electrical signal and the signal from the modulator are combined in the balanced mixer. The output of the balanced mixer creates an error signal from which feedback circuitry may detect changes in the frequency of the laser and make adjustments to the laser frequency to, in effect, stabilize the laser frequency.
This technique has the disadvantage of using the piezoelectric device to electromechanically modulate the FP. It is desirable in a number of circumstances to avoid such active dithering.
Another conventional technique employs a laser, a splitter, an offset control, an FP, a first photodetector, a second photodetector and a differential amplifier. The output from the laser is split such that a portion of the light enters the FP and the remaining portion enters the offset control. The output of the FP enters the first photodetector which creates a first photocurrent. The output of the offset control enters the second photodetector which creates a second photocurrent. The two photocurrents provide input signals for the differential amplifier. The output of the differential amplifier creates an error signal from which feedback circuitry may detect changes in the frequency of the laser and make adjustments to the laser frequency to stabilize the laser frequency. The offset control is used during an initialization process to set a zero output for the differential amplifier when the desired frequency is obtained.
This technique suffers from its dependence upon the differential amplifier and the offset control. If the offset control drifts, then the zero output point drifts and the error signal will not be reliable. Furthermore, the second technique depends heavily upon the stability of the differential amplifier which must be highly reliable and must not change with time.
What is needed is a laser frequency stabilization technique that is passive and highly reliable.
The present invention provides for a laser frequency stabilization system having a laser lasing at a single frequency. The laser has a desired frequency and provides an optical signal including a substantial frequency chirp signal. An optical filter is coupled to the laser and has a frequency response with a resonant peak centered on the desired frequency of the laser. The optical filter filters the optical signal according to the frequency response. A photodetector is coupled to the optical filter and provides a photocurrent in response to the filtered optical signal. A modulator is coupled to the laser and provides a modulation signal which modulates the optical signal of the laser. A derivative module is coupled to the modulator and provides a derivative signal which is a time derivative of the modulation signal. A balanced mixer is coupled to the derivative module and to the photodetector. The balanced mixer provides a mixed signal including a mixing of the photocurrent and the derivative signal.
The present invention also provides for an apparatus for laser frequency stabilization including a laser, a modulator, a derivative module, a Fabry-Perot etalon, a photodetector, a balanced mixer, a signal processing module, and a feedback control module. The laser has an output and lases at a single frequency. The laser has a desired frequency and provides an optical signal at the output of the laser. The optical signal includes a substantial frequency chirp signal. The modulator is coupled to the laser and provides a modulation signal for modulating the laser. The derivative module is coupled to the modulator and provides a derivative signal which is a time derivative of the modulation signal. The Fabry-Perot etalon is coupled to the output of the laser and has a frequency response including a plurality of resonances. One of the plurality of resonances is centered on the desired frequency. The etalon filters the optical signal according to the frequency response. The photodetector is coupled to the etalon and receives the filtered optical signal and provides a photocurrent in response to the filtered optical signal. The balanced mixer is coupled to the photodetector and to the derivative module. The balanced mixer provides a mixed signal by mixing the photocurrent with the derivative signal. The signal processing module is coupled to the balanced mixer and provides an error signal which is a time average of the mixed signal. The feedback control module is coupled to the signal processing module and the laser. The feedback control module uses the error signal in order to adjust the single frequency of the laser.
The present invention also provides for a laser frequency stabilization system including a laser with a desired laser frequency. The laser frequency stabilization system includes an optical signal, emitted from the laser, including a substantial frequency chirp signal; means for filtering the frequency chirp signal according to frequency; and means for providing an error signal from the filtered frequency chirp signal.
The present invention provides for a method for stabilizing laser frequency. The method includes the steps of providing a modulation signal, modulating an optical signal from a laser by the modulation signal, redirecting part of the optical signal including a frequency chirp signal to an optical filter, centering a resonant response of the optical filter around a desired frequency of the laser, filtering the optical signal according to the resonant response of the optical filter, creating a photocurrent from the filtered optical signal, mixing the photocurrent with a time derivative of the modulation signal and time averaging the mixed signal.
The present invention also provides for a process for creating an error signal for a laser frequency stabilization system. The process includes the steps of creating a substantial frequency chirp signal, creating a sloped frequency response around a lasing frequency, filtering the frequency chirp signal according to the sloped frequency response and deriving the error signal from the filtered frequency chirp signal.
The present invention also provides for a method for creating an error signal for a laser frequency stabilization system. The method includes the step of using a frequency chirp signal from a modulated laser to create the error signal.